From 102dd0072d34fa081f5861148be61c0c3739827c Mon Sep 17 00:00:00 2001
From: Daniel Friesel <derf@finalrewind.org>
Date: Thu, 26 May 2022 07:52:52 +0200
Subject: verify checksum, parse automatic baseline correction data

---
 README.md  |  8 ++++----
 init.lua   | 10 +++++-----
 mh-z19.lua | 37 +++++++++++++++++++------------------
 3 files changed, 28 insertions(+), 27 deletions(-)

diff --git a/README.md b/README.md
index 84bd62c..f6fed93 100644
--- a/README.md
+++ b/README.md
@@ -41,17 +41,17 @@ levels at boot time and may therefore be unsuitable for MH-Z19 connection.
 Copy **mh-z19.lua** to your NodeMCU board and set it up as follows.
 
 ```lua
-mhz19 = require("mh-z19")
+mh_z19 = require("mh-z19")
 port = softuart.setup(9600, 1, 2)
 port:on("data", 9, uart_callback)
 
 function uart_callback(data)
-	if sds011.parse_frame(data) then
-		-- mhz19.co2 contains the CO₂ concentration in ppm
+	if mh_z19.parse_frame(data) then
+		-- mh_z19.co2 contains the CO₂ concentration in ppm
 	end
 end
 
-port:write(mhz19.query())
+port:write(mhz19.c_query)
 ```
 
 ## Application Example
diff --git a/init.lua b/init.lua
index 76b8225..b6d7175 100644
--- a/init.lua
+++ b/init.lua
@@ -16,7 +16,7 @@ ledpin = 4
 gpio.mode(ledpin, gpio.OUTPUT)
 gpio.write(ledpin, 0)
 
-mhz19 = require("mh-z19")
+mh_z19 = require("mh-z19")
 
 poll = tmr.create()
 
@@ -58,13 +58,13 @@ function connect_wifi()
 end
 
 function uart_callback(data)
-	if not mhz19.parse_frame(data) then
+	if not mh_z19.parse_frame(data) then
 		print("Invalid MH-Z19 frame")
 		return
 	end
 
-	local json_str = string.format('{"rssi_dbm":%d,"co2_ppm":"%d"}', wifi.sta.getrssi(), mhz19.co2)
-	local influx_str = string.format("co2_ppm=%d", mhz19.co2)
+	local json_str = string.format('{"rssi_dbm":%d,"co2_ppm":"%d"}', wifi.sta.getrssi(), mh_z19.co2)
+	local influx_str = string.format("co2_ppm=%d", mh_z19.co2)
 
 	if not publishing_mqtt then
 		watchdog:start(true)
@@ -94,7 +94,7 @@ function publish_influx(payload)
 end
 
 function query_data()
-	port:write(mhz19.query())
+	port:write(mh_z19.c_query)
 end
 
 function hass_register()
diff --git a/mh-z19.lua b/mh-z19.lua
index fe269b8..0b0a53e 100644
--- a/mh-z19.lua
+++ b/mh-z19.lua
@@ -1,28 +1,29 @@
-local mhz19 = {}
+local mh_z19 = {}
 
-local c_read = string.char(0xff, 0x01, 0x86, 0x00, 0x00, 0x00, 0x00, 0x00, 0x79)
+mh_z19.c_query = string.char(0xff, 0x01, 0x86, 0x00, 0x00, 0x00, 0x00, 0x00, 0x79)
 
-mhz19.co2 = nil
-mhz19.temp = nil
-mhz19.tt = nil
-mhz19.ss = nil
-mhz19.uu = nil
+mh_z19.co2 = nil
+mh_z19.temp = nil
+mh_z19.abc_count = nil
+mh_z19.abc_ticks = nil
 
-function mhz19.query()
-	return c_read
+function checksum(b1, b2, b3, b4, b5, b6, b7)
+	return 0xff - ((b1 + b2 + b3 + b4 + b5 + b6 + b7) % 0x100) + 1
 end
 
-function mhz19.parse_frame(data)
-	local head, cmd, co2h, co2l, temp, tt, ss, uh, ul = struct.unpack("BBBBBBBBB", data)
-	if head ~= 0xff or cmd ~= 0x86 then
+function mh_z19.parse_frame(data)
+	local start, cmd, co2h, co2l, temp, u1, abc_ticks, abc_count, cksum = struct.unpack("BBBBBBBBB", data)
+	if start ~= 0xff or cmd ~= 0x86 then
 		return false
 	end
-	mhz19.co2 = co2h * 256 + co2l
-	mhz19.temp = temp - 40
-	mhz19.tt = tt
-	mhz19.ss = ss
-	mhz19.uu = uh * 256 + ul
+	if checksum(cmd, co2h, co2l, temp, u1, abc_ticks, abc_count) ~= cksum then
+		return false
+	end
+	mh_z19.co2 = co2h * 256 + co2l
+	mh_z19.temp = temp - 40
+	mh_z19.abc_ticks = abc_ticks
+	mh_z19.abc_count = abc_count
 	return true
 end
 
-return mhz19
+return mh_z19
-- 
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